Define listdir in posix and import it in os. (#3258)

stdlib/2/os/__init__.pyi

@@ -3,7 +3,7 @@
 
 from builtins import OSError as error
 from io import TextIOWrapper as _TextIOWrapper
-from posix import stat_result as stat_result  # TODO: use this, see https://github.com/python/mypy/issues/3078
+from posix import listdir as listdir, stat_result as stat_result  # TODO: use this, see https://github.com/python/mypy/issues/3078
 import sys
 from typing import (
     Mapping, MutableMapping, Dict, List, Any, Tuple, Iterator, overload, Union, AnyStr,
@@ -200,7 +200,6 @@ def getcwd() -> str: ...
 def getcwdu() -> unicode: ...
 def chmod(path: _PathType, mode: int) -> None: ...
 def link(src: _PathType, link_name: _PathType) -> None: ...
-def listdir(path: AnyStr) -> List[AnyStr]: ...
 def lstat(path: _PathType) -> Any: ...
 def mknod(filename: _PathType, mode: int = ..., device: int = ...) -> None: ...
 def major(device: int) -> int: ...

stdlib/2/posix.pyi

@@ -1,4 +1,4 @@
-from typing import Dict, List, Mapping, Tuple, Union, Sequence, IO, Optional, TypeVar
+from typing import AnyStr, Dict, List, Mapping, Tuple, Union, Sequence, IO, Optional, TypeVar
 
 error = OSError
 
@@ -7,6 +7,8 @@ environ: Dict[str, str]
 pathconf_names: Dict[str, int]
 sysconf_names: Dict[str, int]
 
+_T = TypeVar("_T")
+
 EX_CANTCREAT: int
 EX_CONFIG: int
 EX_DATAERR: int
@@ -143,8 +145,7 @@ def kill(pid: int, sig: int) -> None: ...
 def killpg(pgid: int, sig: int) -> None: ...
 def lchown(path: unicode, uid: int, gid: int) -> None: ...
 def link(source: unicode, link_name: str) -> None: ...
-_T = TypeVar("_T")
-def listdir(path: _T) -> List[_T]: ...
+def listdir(path: AnyStr) -> List[AnyStr]: ...
 def lseek(fd: int, pos: int, how: int) -> None: ...
 def lstat(path: unicode) -> stat_result: ...
 def major(device: int) -> int: ...